1. Field of the Invention:
The invention relates to a technique for locally increasing the surface heat spreading and through-thickness thermal conductivity of graphite/epoxy laminates. More particularly, the spreading of heat and the heat exchange efficiency of a composite matrix is achieved by employing high thermal conductivity fibers and low thermal conductivity fibers alternately arranged in a stacked configuration.
2. Discussion of the Prior Art
The polymer matrix composite through-thickness thermal conductivity in a composite becomes particularly important in applications such as composite space electronics enclosures where the heat dissipation is entirely dependent on thermal conduction to a heat sink. The spreading of heat at the composite surface and subsequent localized conduction in the through-thickness direction down to high thermal conductivity fiber is thought to be the key to designing a light weight, thermally efficient enclosure. Consequently, there exists a need for lightweight, thermally conductive electronics enclosures for spaceborne applications. However, the thermal conductivity of most carbon fabric/epoxy composites is very low (1-2 W/m.degree.K) compared to aluminum (180 W/m.degree.K) and copper (390 W/m.degree.K).
Authorities list the thermal conductivity of uni-directional graphite/epoxy (Hercules AS-1/3502) as 6.3 W/m.degree.K (longitudinal direction), 1.1 W/m.degree.K (transverse direction) and 0.77 W/m.degree.K (through thickness direction). Other authorities predict the in-plane thermal conductivity for 35% fiber, plain weave graphite/epoxy as 1.5 W/m.degree.K and 0.25 W/m.degree.K in the through thickness direction. Pitch fiber (AMOCO P-120 or K1100) epoxy composites have a longitudinal thermal conductivity of, respectively, 360 W/m.degree.K and 500 W/m.degree.K in uni-directional laminates. However, the through-thickness thermal conductivity is still low, on the order of 1-2 W/m.degree.K. Also known is the use of chemical vapor deposition of diamond coatings with a thermal conductivity of 1800-2000 W/m.degree.K to increase the surface conductivity in microelectronics. Differences in processing temperatures and techniques apparently render the diamond approach as unlikely to be suited for polymer matrix composites.
Low conductivity in the through-thickness direction of uni-directional pitch fiber composites thus poses problems in any design that requires a thermal path to a high thermal conductivity fiber, particularly where large amounts of heat are input over small areas. As one approach, introducing metal particles into the matrix will increase the through-thickness conductivity throughout the composite; however, the weight density of the metallic composite over a polymer matrix composite is greatly increased. Locally increasing the through-thickness thermal conductivity and then allowing the high thermal conductivity fibers to spread and orient the heat flow to a sink appears to be an optimal solution to this problem.